Javascript用于获取父方法的对象

Question

In the following add method of myObj how can I get this inside map? In other words, this when wrapped to map, points to that anonymous function inside map. How can I get this there?

Note: Workarounds like creating a new variable temp_sum and adding and returning are not preferred. Because, I might have to do some tests inside them using the this keyword.

var myObj = {

    sum        : 0,
    toAdd      : [2,3,4],
    add        : function(){

        this.toAdd.map(function(num){
           this.sum += num //<-- How to get this.sum from here           
        })

       return this.sum;

    }


};

var m = Object.create(myObj);
var _sum = m.add();
document.getElementById("test").innerHTML = _sum;

Answer 1

You could use bind

var myObj = {

    sum        : 0,
    toAdd      : [2,3,4],
    add        : function(){

        this.toAdd.map(function(num, index){
           this.sum += num;
        }.bind(this))

       return this.sum;
    }
};

or reduce

var myObj = {

    sum        : 0,
    toAdd      : [2,3,4],
    add        : function(){
        this.sum = this.toAdd.reduce(function(a,b){
           return a + b;
        });

        return this.sum;
    }
};

or a for loop

var myObj = {

    sum        : 0,
    toAdd      : [2,3,4],
    add        : function(){
        for (var i=0; i<this.toAdd.length; i++) {
            this.sum += this.toAdd[i];
        }

        return this.sum;
    }
};

Answer 2

Array.prototype.map method accepts optional argument: object value to be used as this . So your code will become as simple as:

add: function () {
    this.toAdd.map(function (num) {
        this.sum += num;      
    }, this);
    return this.sum;
}